In a printer, a copier or other imaging system, paper or other media is loaded as a stack of cut sheets. For example, blank paper or other recording media is loaded into one or more input trays so that it can be printed. How much media is left in the input tray is not always readily apparent to the user because of the design and location of the input tray. Yet the information of how much media remains is useful for managing the printing operation, as well as for an early warning that more media will be needed to be supplied. As a first example, suppose a user requests a print job requiring 20 sheets of media, but only 10 sheets are in the input tray. If the user leaves the printing job unattended and comes back later, the user will be disappointed to find that the printing job is unfinished because the printer ran out of paper. In addition, while waiting for the job to continue, the printhead may return to the maintenance station and expel additional ink that would not have occurred otherwise. As a second example, if a user has a job that needs to be printed, but does not realize the printer is almost out of paper, the user may need to make a special trip to obtain more paper, thus causing delays in printing the job. In this example, an early warning would be helpful so that the user can obtain more paper before his local supply runs out.
Media stack height detectors have been disclosed in the prior art, for example U.S. Pat. No. 5,839,015 and U.S. Pat. No. 7,374,163. However, competitive pressures make it desirable to incorporate the function of media stack height detection at low cost. (The term “stack height” is used generically herein to refer to an amount of media in a stack of media and does not necessarily imply a vertical measurement.) Prior art media stack height detectors typically use an extra coupling component that contacts the top sheet of the media stack at one end and has a portion that provides a stack-height-dependent signal via a corresponding sensor at another other end. The extra coupling component not only adds cost to the system, but also its contacting the sheet of media adds drag as the sheet is moved from the media input tray into a position for printing. U.S. Pat. No. 7,828,282, the disclosure of which is herein incorporated by reference, addresses this problem by affixing a member having a height-dependent property to the pick arm assembly that houses the pick roller. Since the pick roller contacts the first sheet in the stack, a sensor that measures the height-dependent property is able to determine the position of the first sheet in the stack.
It has been found that, under certain circumstances, the accuracy of media stack height detectors can be limited to about plus or minus five sheets of media. Although this is sufficiently accurate for providing an early warning to the user that the media holding receptacle is approaching empty, a more accurate indication is preferable for the function of notifying the user whether there is enough media to complete a particular print job. It is also sometimes found that the accuracy of stack height detection becomes even worse as the media holding receptacle becomes closer to empty. Thus, at the times when improved accuracy is needed, the accuracy actually becomes worse. Furthermore, if media in the media holding receptacle has already been printed on, it can become less flat, resulting in even worse accuracy in the measurement of the amount of media in the stack.
What is needed is a method of measuring the amount of media in a stack of media having improved accuracy, and an accuracy that does not degrade as the media holding receptacle approaches empty.